When cathodic protection is used to protect marine structures and structures in water a variety of sacrificial and impressed current anode systems are used. Impressed current anode systems are used for applications where higher DC currents are required and for retrofits of already existing facilities such as offshore oil platforms and the wetted portions of steel and other metallic structures.
The prior art for impressed current cathodic protection marine anodes has generally been known as anode sleds. The basic concept has been to use standard anodes, such as those used for non-marine applications in their existing form and to mount the anodes on a weighted, e.g., concrete, sled of some sort. The anodes have been as simple as steel railroad rails and in recent times silicon iron anodes, graphite anodes, platinum coated anodes and mixed metal oxide anodes. The anodes have generally been in tubular form, with some use in plate form. The prior art anodes are connected to one or more cables, and because of the shape and construction of the anodes, the connection to the cables generally must be done in a factory before the anode is mounted to the sled. Most of the prior art anodes must be fully assembled and in some cases the concrete weight and support material must be cast before the anode assembly is shipped from the factory. The requirement to connect the cable and possibly cast the concrete increases the cost and shipping of the anode, and limits the flexibility of the anode cabling.
The assignee of this invention, Matcor, Inc., of Doylestown, Pa., has provided various anode assemblies for marine applications. Such assemblies are referred to as Sea-Bottom anodes and Sea-Floor anodes and use mostly solid rod and tubular anodes mounted in a vertical or horizontal direction. The anodes are part of assemblies that contain the anode to cable connections and the concrete weight material. While the concrete material can be cast in the field, it is more difficult and factory connections are recommended. The finished weight of the anode sleds can be from 1,000 to over 5,000 pounds.
Prior art anode sleds generally are more desired in heavy weights to prevent the anode sled from shifting or moving on the sea floor. If the anode sled moves easily, it can be moved great distances from the structure to be protected and damage or sever the power cable to the sled. Another concern with the marine anode sled is keeping the active anode above the sea bottom. If the anode sled sinks or is covered with mud or sand, the performance of the anode will be affected and the protective DC current may not go to the structure intended to be protected.
While the foregoing anode systems perform well, there are limitations as to their performance and durability. There are many applications where DC current output requirements can be several hundred to one thousand or more amperes of DC current. The current output of conventional tubular, rod or plate anodes is limited to the surface area of the anode. To compensate for the current limitation for each anode, more anodes and longer anodes must be used. However, additional anodes cannot be spaced too close together without creating interference between the anodes. To space the conventional anodes further apart requires larger anode sled assemblies. As a result of the foregoing, the general convention is to use additional anode sleds.
Another limitation of conventional sled-type anode assemblies is the physical resistance to the elements in the marine environment. In particular, when the prior art anode sleds are placed on the sea floor and are subjected to intense water currents, debris and ice. The DC current requirements may require an anode surface area larger than any one tubular shaped anode and it is not unusual to have two, three or more mixed metal oxide anodes, each measuring one inch in diameter and up to five feet long. The mountings for these anodes and the concrete platform needed to hold the anodes can be large and have great resistance to the water currents and therefore are subject to damage by debris and tidal action. To keep the anodes out of and above the mud on the bottom of the sea, some sleds have structures to elevate the anodes. These structures are subject to moment arm damage or float freely on a tether and the stresses with this type of installation can also cause failure.
Thus, there presently exists a need for marine anodes which overcome the disadvantages of the prior art. The subject invention addresses that need.